This invention relates to an improved dielectric ceramic composition of the X7R variety, and more particularly to an improved such composition which is capable of exhibiting a dielectric constant (K') of greater than 5,000. Even more particularly, this invention relates to an improved ceramic capacitor made from such material.
The art relating to the production of ceramic dielectric compositions for use in producing monolythic ceramic capacitors is extremely highly developed. One reason for such a plethora of prior art is that, with even the slightest apparent variations in the components or ratios of components in a dielectric composition, the resultant changes in the characteristics of a ceramic capacitor are often new and unexpected. In connection with the invention described hereinafter, it has been discovered that the dielectric constant of a barium titanate (BaTiO.sub.3) based material can be increased substantially, and surprisingly, by utilizing a novel ternary system.
At the outset, it should be noted that the prior art has disclosed various temperature-stable barium titanate compositions which are claimed to produce a dielectric constant in the range of 3000 to 4700. U.S. Pat. No. 4,882,305, for example, suggests that a barium titanate mixture containing cobalt oxide and niobium pentoxide, can produce multi-layer ceramic capacitors which meet the X7R rating of the U.S. Electronics Industries Association (EIA) Standard--i.e., its temperature coefficient of dielectric constant (K') varies no more than about .+-.15% from the reference value at 25.degree. C. over a temperature range of -55.degree. C. to 125.degree. C.
However, the compositions disclosed in the above-noted patent yielded either lower than desired insulation resistance values, or higher dissipation factors than desired for most commercial applications.
Compositions disclosed in U.S. Pat. No. 4,816,430 which issued a short time before the 4,882,305 patent, also disclosed temperature-stable barium titanate compositions which exhibit dissipation factors below about 1.8%, and insulation resistance values above about 10,000 ohm-farads at 25.degree. C., and 1,000 ohm-farads at 125.degree. C. The problem with these compositions, however, is that their dielectric constants are reduced to about 4,000 whenever the X7R temperature characteristic is achieved.
Further refinements of these compositions are disclosed in U.S. Pat. No. 4,939,108, which suggests that even greater dielectric constants can be achieved by improving the homogeneity of the dopant distribution in the barium titanate composition. Although this latter patent suggests that dielectric constants in excess of 4900 can be achieved, the process for producing the composition appears to be very impractical. Removal of undesirable chlorides is difficult at best, and expensive washing and pollution control steps would become necessary to commercialize such a process.
The homogeneity theory advanced in the above-noted U.S. Pat. No. 4,939,108, conflicts with the conclusions reached by Hennings and Rosentein (Temperature-Stable Dielectric based On Chemically Inhomgeneous BaTiO.sub.3, Apr., 1984, Vol. 67, No. 4 of Journal of the American Ceramic Society), who concluded from tests that they conducted that X7R--type dielectric ceramics owe their TC characteristics, not only to small grain size, but to the nature of the chemically inhomogeneous materials that are employed. In his article entitled Barium Titanate Based Ceramic Materials For Dielectric Use, Int. J. High Technology Ceramics 3, pp. 91-110, 1987 Elservier Applied Science Publishers Ltd., England, Hennings recognized that an increase in the dielectric constant of barium titanate compositions can be associated with a decrease in grain size, and again noted that high dielectric constant, temperature-stable materials exhibit chemically heterogeneous grains, rather than homogeneous grains. However, as will be noted hereinafter, none of the foregoing prior art teachings suggests the novel ternary system disclosed hereinafter, and the unexpectantly high dielectric constant which is achieved by such a composition.
The closest prior art known to the applicant appears to be that of a Soviet publication entitled BaTiO.sub.3 -Based Solid solution of Niobates And Tantalates Of Transition Metals, Soviet Phys. Solid State 2, 66 (1960), translated from Fizika Tverdogo Tela Vol. 2, No. 1, pp. 73-79, Jan., 1960. This article discloses or suggests the use of solid solutions composed of barium titanate and metaniobates, pyroniobates, and tantalates of Mn, Ni, and Co to produce very high dielectric constant, non-temperature stable materials. However, conventionally calcined barium titanate was employed with the various stoicheometric niobates and tantalates, thus producing bodies which would not be acceptable for use as X7R dielectrics. Moreover, the Soviet article discusses only two component systems--i.e., barium titanate and a single niobate. (See FIGS. 1-4 of the Soviet publication). No. attempt is made to either incorporate non-stoicheometric niobates or tantalates, or combinations thereof, as is the case with the novel ternary system disclosed herein.
Accordingly, it is an object of this invention to provide an improved, inexpensively processed dielectric ceramic composition which qualifies as an X7R type dielectric, and which is capable of resulting in dielectric constants substantially greater than 5000.
Still another object of this invention is to produce an improved dielectric ceramic composition utilizing normal processing steps on a novel barium titanate based ternary system.
A further object of this invention is to produce improved multi-layer capacitors from the novel composition disclosed herein.
Other objects of the invention will be apparent hereinafter from the specification and from the recital of the appended claims, particularly when read in conjunction with accompany drawing.